Hitherto, an Sn—Pb solder alloy has often been used for soldering various apparatuses because it has advantages such that a melting point is low, wettability is good even in an oxidizing atmosphere, and the like. However, since lead has toxicity, in recent years, the realization of a lead-free solder has rapidly been progressed from a viewpoint of prevention of environmental pollution in association with a disposing process or the like of electronic apparatuses. However, since a lead-free solder alloy is inferior to the conventional Sn—Pb solder alloy in terms of the wettability, melting point, costs, and the like, it is not developed yet at present as a perfect substitute.
In such a stream where importance is attached to the environment, the realization of a lead-free solder is an emergency subject also in reflow soldering as one of mounting steps of electronic circuit devices. Therefore, hitherto, an Sn—Zn solder alloy containing about 9 wt % zinc (eutectic composition) has been proposed as a lead-free solder material which is expected to be put into practical use for reflow soldering. Further, an Sn—Zn solder alloy containing about 8 wt % zinc and 1 to 3 wt % bismuth has been also proposed. Those Sn—Zn solder alloys have advantages such that a eutectic temperature of a tin-zinc alloy is equal to 199° C. closest to a eutectic point of a tin-lead alloy among Sn-based lead-free solder alloys, and costs of raw materials of them are lower than those of the other lead-free solder alloys.
Generally, solder paste which is used for the reflow soldering is made of a mixture of solder particle and flux. An activator to clean the surface of a soldering target and improve the wettability of the solder during soldering has been added to the flux. Therefore, in the Sn—Zn solder alloy which is inferior to the conventional Sn—Pb solder alloy in terms of the wettability, it cannot help enhancing the activator particularly in order to improve the wettability of the solder. Moreover, in the Sn—Zn solder alloy, zinc-riched phases exist on the surface and the inside of the manufactured solder particle.
Therefore, in the conventional Sn—Zn lead-free solder alloy, there are problems such that since zinc is an active element, while the solder paste which is made by mixing the solder powder with the flux is preserved, zinc on the surface of the solder particle easily reacts on the activator in the flux, so that viscosity of the solder paste is rapidly and remarkably increased by its reactive product more than the conventional Sn—Pb solder alloy, and when such a solder paste is used, for example, it is impossible to print onto a printed wiring board.
There is also a problem such that if an activity or an amount of the activator is suppressed in order to prevent an increase in viscosity of the solder paste, a zinc oxide which is produced during soldering cannot be reduced, so that wettability of the solder deteriorates. As mentioned above, hitherto, it is extremely difficult to reconcile the wettability and preservation stability for a long time of the solder, and that causes non-spread of the lead-free solder paste which is effective to protect the environment.
Therefore, hitherto, a method of adding another element, for example, aluminum has been disclosed as means for solving the above problem (International Publication No. WO02/34969A1). However, since the method of adding aluminum has a problem of occurrence of an unmelted solder as will be explained hereinlater, it cannot help limiting the amount of added aluminum to a narrow range in a present situation.
As mentioned above, there is an Sn—Zn solder alloy containing bismuth as conventional lead-free solder alloys. In such a kind of solder alloy, there is a tendency such metallic gloss on the surface of the solder after solidification becomes dim. Therefore, besides the reconcilation between the wettability and the preservation stability, it is also demanded to solve the problem on the external appearance.
It is, therefore, an object of the present invention to provide an Sn—Zn lead-free solder alloy, its mixture and a soldered bond using the same in which wettability of solder is improved and preservation stability for a long time of solder paste is also excellent and also to improve surface gloss after solidification in the case of an Sn—Zn solder containing bismuth.